We plan to continue our studies of structural and functional properties of neurons and their significance for the processing of sensory information. The electrosensory system of gymnotiform fish serves as a model system for this research. We will explore relationships between the structure and function of neurons by intracellular labelling of physiologically identified cells. Using small, local iontophoretic applications of transmitters or their respective antagonists and by simultaneous extracellular recording from single neurons, we intend to study network properties that are due to the actions of specific transmitters. We will focus on three central nervous structures, the prepacemaker nucleus (PPN) of the midbrain, the pacemaker nucleus (PN) of the medulla, and the electrosensory lateral line lobe (ELL) of the hindbrain. The PPN contains two functionally different premotor systems to control the discharge frequency of the PN, one for inducing smooth changes in frequency, and one for eliciting abrupt, transient accelerations, or 'chirps'. Both forms of frequency modulations occur in response to electric signals of neighbors. Chirps, however, are exclusively produced by animals high in the rank order. Their occurrence in males is facilitated by testosterone and inhibited by serotonin. Serotonin inhibitors, such as methysergide, therefore, promote chirping. By small, local applications of agonists and antagonists to the PPN and its afferent structures, and by simultaneous recording from single neurons related to chirping, we want to identify the underlying structures as well as the mechanisms of neurotransmission in this system. The ELL is the first processing station for electrosensory information. It contains neurons with receptive fields on the body surface. The spatial and temporal dynamics of these fields are modulated by descending recurrent input. The mechanism of this modulation can be studied at the single-cell level by local application of transmitters and their antagonists.